The present invention relates to determining a measurement error of a measurement device.
Measurement devices, such as for example for optical surveying generally include an arrangement of optical elements are, that may me be used to aim at a target. Based on associated measurement scales angles, positions or other measurements can be carried out, for example related to the position of the object relative to the measurement device.
Such measurement devices generally include optical instruments, for example tachymeters, video-tachymeters or total stations. The optical arrangement of the measurement instrument may be a telescope allowing to aim at an object with the help of an ocular. The optical instrument may also include a camera or video camera, to obtain a representation of the object.
FIG. 11 illustrates in an exemplary manner such a measurement device, generally designated by reference numeral 1100. The optical arrangement 1101 of the measurement device is rotatable in a vertical plane around a preferably horizontal axis 1150, the so-called tilting axis or trunnion axis, so that it is possible to aim at objects in different elevations with respect to the measurement device. Besides this the optical arrangement 1101 is pivotable around a preferably vertically oriented axis 1151, also vertical axis, such as on a tripod plate 1102, enabling aiming at objects in different horizontal directions from the measurement device 1100. Analog or digital measurement scales are provided at the measurement device for example for the measured angles. Furthermore means for measuring or estimating a distance to the object may be provided.
Measurements on objects need to be carried out with highest accuracy to obtain the required accuracy for example in construction applications.
The measurement devices in the real world, however, are not infinitely accurate, such that each measurement on an object leads to measurement results that are affected by errors. These errors may perpetuate into the further processing and may lead to problems.
Due to manufacturing tolerances the rotational axes of the measurement device do not run fully vertical to one another, but only with certain deviations. The deviation of the angle between the tilting axis and the vertical axis from 90° generally is termed tilting axis error. The deviation of the angle between the tilting axis and the aiming axis, i.e. line of sight from 90° is generally termed side collimation error. Herein the aiming axis/line of sight is a predetermined optical axis of a telescope or other optical arrangement of the measurement device.
When measuring and angle to an object using the measurement device, these the deviations between the actual angle to the object and the angle indicated by the measurement device, that is the tilting axis error and the side collimation error, leads to an angle and rapport a measurement error, for example in horizontal or vertical direction. Therefore the measurement units often are adjusted or calibrated upon starting operation or during operation in certain intervals.
According to a known method for calibrating the measurement scales, the measurement device is first deployed and then oriented towards an object. Then the respective angle measurements are carried out in a so-called first face or face orientation, for example a measurement of a horizontal angle and a particular angle to towards the object. Then the optical device, for example the telescope of the measurement device, as illustrated in FIG. 11 by reference numeral 1101, is rotated by 180° around the vertical axis 1151, and furthermore by an angle of 360° minus a vertical angle, for example the zenith angle, between the telescope and be vertical axis, around the horizontal axis 1150. This process of internally tilting the measurement device is termed to snap through the measurement device. In this process the measurement device or rather the optical arrangement of the measurement device is transferred from a first face, in which the first measurement of the object was carried out, into a second to face. In the second face the second measurement of the object is carried out.
From the angle measurements in the first face and the second face of the tilting axis and the side collimation error may be derived. Calibration can be carried out by correcting the measurement scales of the measurement device on the basis of the tilting axis error and the side collimation error, such that for each subsequence measurement the measurement error is eliminated. A calibration may be carried out once, or before a series of measurements, or intervals between measurements.
Furthermore an object may generally also be measured in both face orientations of the instrument, such that for each measurement the errors may be corrected.
Calibration may, the more frequent it is carried out, require a considerable amount of time, which should be avoided.